The present invention relates in general to heavy cyclical centrifugal machines and, more particularly, to an apparatus for controlling the speed and direction of a rotating centrifugal basket of the machine. While the present invention is generally applicable to heavy cyclical centrifugal machines, it will be described herein with reference to batch centrifugal machines used for manufacturing and refining sugar.
A centrifugal machine uses centrifugal force to separate substances, such as, for example a liquid component (the filtrate) from a solid component (the cake), in a slurry which has been introduced to the centrifugal machine. A filtering perforate wall traps the cake by a filter, whereas the filtrate passes through the filter.
A problem encountered when operating heavy cyclical centrifugal machines of the type used to manufacture and refine sugar is the inaccurate control of the speed of rotation of centrifugal baskets of the machines. These baskets should be fully loaded to their maximum capacities to maximize the productivity of the machines. Unfortunately, should the rotation of the centrifugal basket inadequately dispel the filtrate, the cake may be compromised. Variations in the loading properties of the charge material, massecuite for sugar manufacture and refining, can affect the efficiency of cycle to cycle centrifugal processing. Since these variations in loading properties are difficult or impossible to control, it has been an ongoing goal in the industry to control the motor operations of centrifugal machines such that the machines may be loaded with maximum charge in spite of the charge material variations.
The operational speeds of a heavy cyclical centrifugal machines are known to be established through the use of 2-speed motors, which utilize a dual set of internal windings such that the motor may operate at either a low or a high speed. However, a portion of a typical centrifugal machine cycle may require the rotational speed of the basket to be maintained at some intermediate value on the low speed windings. One known method of accomplishing this task is to repeatedly open and close a set of electrical contacts that energize and de-energize the low speed windings. This causes wear on the electrical components and may require frequent maintenance.
Further, it is a practice to reverse the direction of rotation of the centrifugal machine basket while discharging the charge material from the centrifugal machine basket. This is typically implemented by mechanically braking the rotation of the centrifugal machine basket until the centrifugal machine basket is at rest. The main 2-speed motor is electrically disengaged, and a second motor is engaged to rotate the centrifugal machine basket in the reverse direction. Upon completion of the discharge phase of the centrifugal machine cycle, the second motor disengages and the 2-speed motor re-engages to start a new cycle. Thus, the cost of the centrifugal machine is increased, and the motor control circuitry is complicated by the need to switch between multiple motors during each cycle.
Additionally, peak power demands, which occur typically during accelerating the centrifugal basket, can cause considerable power drain. This is because engaging the 2-speed motor low or high speed windings amounts to xe2x80x9cacross-the-linexe2x80x9d starting of the motor. This has the effect of huge current demand on the electrical transformer during motor acceleration. The power drawn during operation affects the refiners ability to process sugar cost efficiently.
Accordingly, there is a need for an improved motor control for a centrifugal machine that eliminates the need for a second motor for operating the basket is a reverse direction, and reduces the peak power drawn by the centrifugal machine.
The present invention overcomes the disadvantages of previously known motor controllers for centrifuge machines wherein a motor controller is provided for a centrifuge machine including a logic control module, one or more power cells, and one or more contactors. The logic control module is capable of interfacing with the main centrifuge controller and provides control over the power cells and contactors to provide a voltage ramp-up to accelerate the centrifuge basket. As such, the logic control module avoids the current draining problems associated with across the line starting of the centrifuge motor. The power cells receive a voltage from the main power supply, and output to the contactors variable power to control centrifuge motor speed. Further, the configuration of multiple contactors to reverse the power supplied to the centrifuge motor windings may eliminate the need for a second, reverse direction motor.
In accordance with one embodiment of the present invention, a motor controller for a centrifuge machine comprises a first power cell having an input coupled to a main power supply, and an output. The first power cell is switchable between an on state where power is supplied to the output, and an off state where no power is supplied to the output. The motor controller also comprises a first contactor connected between the output of the first power cell and first windings of a motor. The first contactor is switchable between a first state, wherein an electrical connection is made between the first power cell and the motor, and a second state wherein an electrical connection is broken between the first power cell and the motor. Additionally, the motor controller comprises a logic control module coupled to the first power cell and the first contactor. The logic control module is arranged to interface with the controls of the centrifuge machine to selectively apply and vary power to the motor. Power is supplied to the motor when the logic control module switches the first contactor to the first state to establish an electrical connection between the first power cell and the motor. The logic control module further communicates with the first power cell to vary the power output by the first power cell, and accordingly adjusts the power to the motor thereby controlling the rotation of the centrifuge. For example, where the first power cell is implemented as a pair of silicon controlled rectifiers (SCRs), the logic control module controls the amount of power the first power cell supplies to the motor by varying the rate at which the logic control module turns the first power on and off.
When used with certain heavy duty cylindrical centrifugal machines, three phase AC power may be required to power the motor. Under such circumstances, the motor controller further comprises second and third power cells. The power supply comprises a three phase power supply and each of the first, second and third power cells couple a respective phase of the three phase power supply to the first contactor.
Further, more elaborate motor control schemes may be realized by incorporating into the motor controller a second contactor connected between the first power cell and second windings of the motor. The second contactor is switchable between a first state, wherein an electrical connection is made between the first power cell and the motor, and a second state wherein an electrical connection is broken between the first power cell and the motor. The second contactor is coupled to the logic control module. The logic control module is further arranged to control the first and second contactors for selectively supplying power to the first and second windings of the motor. For example, the motor may be a 2-speed motor having first windings, which are low speed windings connected to the first contactor. The second windings may be high speed windings connected to the second contactor. The logic control module is arranged to switch both the first and second contactors into their respective second states, thus the motor controller supplies no power to the motor. By maintaining the second contactor in the second state, and turning the first contactor to the first state, the power cell is coupled to the first (low speed) motor windings, and isolated from the high speed windings. The logic control module may control the speed of the motor by varying the power delivered to the low speed windings via the power cell. In contrast, where high speeds of centrifuge rotation are required, the logic control module switches the first contactor to the second state isolating the low speed windings from the power cell, and transitions the second contactor to the first state, thereby coupling the power cell to the high speed motor windings. The logic control module may optionally switch off the power cell prior to changing the state of either the first or second contactors to avoid switching the contactors while energized.
A third contactor may optionally be connected between the first power cell and the first windings of the motor, the third contactor is switchable between a first state, wherein an electrical connection is made between the first power cell and the motor, and a second state wherein an electrical connection is broken between the first power cell and the motor, the second contactor coupled to the logic control module. The third contactor is wired in parallel with the first contactor and arranged to supply power to the motor such that the motor rotates in a direction opposite of the direction the motor rotates when powered through the first contactor.
Further, the motor controller incorporates the first power cell to adjust the power delivered to the motor while accelerating the motor. The main power supply may supply power to the motor while the motor is rotating at full speed, or alternatively, the motor control may utilize the power cell to power the motor throughout the entire centrifuge cycle.
To more accurately control the motor, the motor controller may optionally include a speed determinative device connected to a first input of the logic control module. The speed determinative device may be a tachometer for example. When using a speed sensing device such as a tachometer, sophisticated programming of the motor controller may be realized. For example, a predetermined speed band may be programmed into the logic control module. During at least a portion of a cycle of operation, the motor speed may be adjusted so that the rotation of the centrifuge is maintained within the speed band. For example, during loading, it the rotation may be maintained at a speed suitable to centrifuge the material being processed.
Additionally, the motor controller may include a voltage suppression device arranged to prevent voltage spikes from reaching the first power cell. For example, a varistor may be used to absorb voltage spikes and transients. Likewise, a current sensing device such as a transformer may be connected to the logic control module to monitor current draw by the motor.
In accordance with another embodiment of the present invention, a centrifuge comprises a basket arranged to receive materials for processing. A motor interconnects to the basket to provide basket rotation in both a forward and reverse direction. A motor controller is coupled to the motor for providing control of the motor, including direction of rotation and rotational speed. The motor controller comprises at least one power cell coupled to a main power supply arranged to control a voltage applied to the motor. The voltage adjusts the rotational speed of the motor. A first contactor couples the power cell to the motor. The first contactor is switchable between a first state wherein an electrical connection is made between the power cell and the motor, and a second state wherein an electrical connection is broken between the at power cell and the motor. A logic control module is coupled to the power cell and the first contactor. The logic control module is arranged to selectively apply and vary power to the motor. For heavy duty cyclical centrifuges, the voltage is a three phase voltage. The motor controller further comprises three power cells, one power cell arranged to control an associated one phase of the three phase voltage.
The motor controller communicates with the power cell to produce a voltage ramp-up to accelerate the basket. The motor controller adjusts the speed of rotation of the basket by selectively turning on and off the power cell. To better adjust the speed of the basket, the motor controller may optionally include a speed determining device coupled to the logic control module. For example, the speed determining device may comprise a tachometer. The tachometer utilizes for example, a magnetic pickup positioned to sense the speed and direction of a toothed gear mounted on a shaft of the motor. The tachometer sends speed control data to a tachometer control unit, the tachometer control unit forwards the information to the logic control module.
The motor controller further comprises a second contactor coupling the power cell to the motor. The second contactor is switchable between a first state wherein an electrical connection is made between the at least one power cell and the motor, and a second state wherein an electrical connection is broken between the at least one power cell and the motor. The second contactor is arranged such that, when the voltage is applied to the motor through the second contactor, the rotation of the motor is opposite the rotation of the motor when the voltage is applied to the motor through the first contactor.
The motor controller may further include a third contactor coupling the power cell to the motor. The third contactor is switchable between a first state wherein an electrical connection is made between the at least one power cell and the motor, and a second state wherein an electrical connection is broken between the at least one power cell and the motor. The motor comprises high speed windings and low speed windings, the first contactor is connected to the low speed windings and the second contactor is connected to the high speed windings.
Additionally, the motor controller may include a voltage suppression device arranged to prevent voltage spikes from reaching the first power cell. For example, a varistor may be used to absorb voltage spikes and transients. Likewise, a current sensing device such as a transformer may be connected to the logic control module to monitor current draw by the motor.
According to yet another embodiment of the present invention, a motor controller for controlling a three phase, 2-speed AC motor comprises three power cells, each of the three power cells connected to a different one phase of a three phase power supply. A first contactor is connected between each of the three power cells and first windings of the 2-speed AC motor, and is arranged to bias the 2-speed AC motor to operate in a first direction. The first contactor is switchable between a first state wherein an electrical connection is made between the at three power cells and the 2-speed AC motor, and a second state wherein an electrical connection is broken between the three power cells and the 2-speed AC motor. A second contactor is connected between each of the three power cells and the first windings of the 2-speed AC motor, in parallel with the first contactor. The second contactor is switchable between a first state wherein an electrical connection is made between the at three power cells and the 2-speed AC motor, and a second state wherein an electrical connection is broken between the three power cells and the 2-speed AC motor. The second contactor is arranged to bias the 2-speed AC motor to operate in a second direction. A third contactor is connected between each of the three power cells and second windings of the 2-speed AC motor. The third contactor is switchable between a first state wherein an electrical connection is made between the at three power cells and the 2-speed AC motor, and a second state wherein an electrical connection is broken between the three power cells and the 2-speed AC motor. The third contactor is arranged to bias the 2-speed AC motor to operate in the first direction on the high speed windings. A logic control module is connected to the three power cells and the first, second and third contactors, arranged to control the amount of power the three power cells supply to the motor. A speed determining device is coupled to the logic control module, the speed determining device arranged to provide data concerning the rotational speed of the 2-speed AC motor to the logic control module.